Qt nested QGraphicsItem coordinates

Question

I would expect, from documentation, that this snippet

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    QGraphicsView *v = new QGraphicsView(new QGraphicsScene);
    setCentralWidget(v);

    QGraphicsRectItem *r1 = v->scene()->addRect(QRectF(100,100, 200,200), QPen(), QBrush(Qt::red));
    QGraphicsRectItem *r2 = new QGraphicsRectItem(r1);
    r2->setRect(QRectF(10,10, 50,50));
    r2->setBrush(QBrush(Qt::blue));
}

would draw a blue rect inside the red one, instead here is the result:

Do I miss something ?

Answer 1

Do I miss something?

Yes.

Calling setRect() on a QGraphicsRectItem does not actually change its pos(), it just changes the position of the rectangle that it draws, but the item's position is unchanged. Thus, your r1 has a pos of (0, 0), but its rectangle is drawn at a (100, 100) offset to its pos().

To avoid confusion, it's usually easier to leave your rect at (0, 0) and call setPos() on your QGraphicsRectItem to move it where you want it.

Answer 2

If you wanted the blue rectangle inside the red one, you could either set its rect position in screen coordinates, or set the blue rectangle as the parent of the red rectangle with setParentItem. That way, the child's position is set relative to its parent.